Polyester fiber dyeable under normal pressure and process for the production thereof

Abstract

A polyethylene terephthalate fiber capable of being dyed under normal pressure and having an initial modulus of more than 50 g/d, a peak temperature (T max) at peak of dynamic mechanical loss tangent (tan δ) measured with a frequency of 110 Hz of about 85° C. to about 110° C., and a peak value of the dynamic mechanical loss tangent ((tan δ) max) of about 0.115 to about 0.135. The fiber has a sheath-core structure such that refractive indices are different between an outer layer of the fiber and an inner layer of the fiber, and a local average refractive index distributed symmetrically around the center of the cross section of the fiber. The fiber is made by extruding a dope of polyester, passing the extruded filaments through a heating zone provided at the surface of the extrusion nozzle and having a length of at least about 5 cm and a temperature of about 150° C. to about the melting point of the polymer, applying a vacuum with an aspirator located below the heating zone, and then winding at a winding speed of at least about 5,000 m/min.

What is claimed is: 
     
       1. A fiber consisting essentially of polyethylene terephthalate capable of being dyed under normal pressure and having an initial modulus of more than about 50 g/d, a peak temperature (T max) at peak of dynamic mechanical loss tangent (tan δ) measured with a frequency of 110 Hz of about 85° C. to about 110° C., a peak value of the dynamic mechanical loss tangent ((tan δ) max) of about 0.115 to about 0.135, and a local average refractive index distributed symmetrically around the center of the cross section of the fiber. 
     
     
       2. A fiber according to claim 1 having an average refractive index (n∥.sub.(0)) of about 1.65 to about 1.70. 
     
     
       3. A fiber according to claim 1 having a difference of average refractive index (Δ∥.sub.(0.8-0)) between an average refractive index (n∥.sub.(0)) and a refractive index at a position 0.8 times from the center of the cross section of the fiber (n∥.sub.(0.8)) of about 10×10 -3  to about 80×10 -3 . 
     
     
       4. A fiber according to claim 1 or 3 wherein the Δ∥.sub.(0.8-0) is about 10×10 -3  to about 40×10 -3 . 
     
     
       5. A fiber according to claim 1 wherein (tan δ) max is about 0.130 or less. 
     
     
       6. A fiber according to claim 1 further having a crystallinity (Xc) of about 70% to about 85%, an apparent crystallite size at a face of (010) (ACS) of about 50 Å to about 75 Å, and a degree of crystal orientation (Co) of about 90% to about 98%.